Aircraft fuel vent pipe

ABSTRACT

An aircraft fuel vent pipe with a pipe body containing a duct which extends along a duct axis between an open end and a closed end. A burst disc closes the closed end of the duct. One or more devices are provided in the duct, the device(s) being positioned or shaped to enable the burst disc to be visually inspected by looking along the duct through its open end, and to modify a sectional profile of the duct transverse to the duct axis so that the sectional profile of the duct changes between different stations along the duct axis. The one or more device(s) inhibit the formation of standing acoustic waves in the duct.

FIELD OF THE INVENTION

The present invention relates to an aircraft fuel vent pipe, and amethod of inhibiting the formation of standing acoustic waves in anaircraft fuel vent pipe.

BACKGROUND OF THE INVENTION

FIG. 18 illustrates an aircraft fuel vent pipe 40. If the aircraft fueltank is overfilled with fuel then the vent pipe 40 is arranged to enableexcess fuel to flow out of the fuel tank. For this reason the vent pipe40 extends downwardly and lies flush with the aerodynamic outer surfaceof the lower skin of the aircraft wing. Thus the open end of the pipe isexposed to an aerodynamic cross flow of air 41 during flight of theaircraft.

The cross flow generates an instability 42 which generates a strongacoustic tone which emanates from the pipe. It is excited by the naturalfrequency of the external flow as a function of the local true air speedand the diameter of the pipe. The resonance that occurs is a function ofthe depth/diameter ratio of the pipe and represents a standing pressurehalf wave 43.

SUMMARY OF THE INVENTION

A first aspect of the present invention provides an aircraft fuel ventpipe comprising a pipe body containing a duct which extends along a ductaxis between an open end and a closed end; a burst disc which closes theclosed end of the duct; and one or more devices in the duct, thedevice(s) being positioned or shaped to enable the burst disc to bevisually inspected by looking along the duct through its open end, andto inhibit the formation of standing acoustic waves in the duct.

A second aspect of the invention provides an aircraft fuel vent pipecomprising a pipe body containing a duct which extends along a duct axisbetween an open end and a closed end; a burst disc which closes theclosed end of the duct; and one or more devices in the duct, thedevice(s) being positioned or shaped to enable the burst disc to bevisually inspected by looking along the duct through its open end, andto modify a sectional profile of the duct transverse to the duct axis sothat the sectional profile of the duct changes between differentstations along the duct axis. Typically the sectional profile of theduct either has different shapes at the different stations, or a rotatedversion of the same shape at the different stations.

The one or more devices may comprise a device with a sectional shape oran angular position about the duct axis which changes between thedifferent stations along the duct axis. For example the device may havean angular position about the duct axis which changes along the ductaxis to form a spiral shape. In one embodiment the spiral shape rotatesaround three-quarters of a full circle (that is, 270 degrees) along itsfull length although other ranges of rotation may be possible.Preferably the pitch of the spiral is not an integer function of thelength of the duct (so the full length of the spiral does not rotatearound 360 degrees or 720 degrees for example).

Optionally the one or more devices comprise first and second deviceswhich are located at different angular positions about the duct axis andoffset along the duct axis so that the sectional profile of the duct ismodified at a first station along the duct axis by the first device butnot the second device, and the sectional profile of the duct is modifiedat a second station along the duct axis by the second device but not thefirst device. Optionally the first and second devices have a spiralshape or sectional shape which changes along the length of the duct.Alternatively the first and second devices may be planar.

The (or each) device typically extends or protrudes into the duct,optionally from an inner wall of the pipe body. The (or each) device maybe attached to the pipe body—for instance it may be bonded or welded tothe pipe body or formed integrally with the pipe body.

The pipe body may consist of a single pipe which provides an inner wallfrom which the device extends or protrudes into the duct. Alternativelythe pipe body may comprise an outer pipe, and an insert which isinserted into the outer pipe and carries the (or each) device—in otherwords the insert provides the inner wall from which the device extendsor protrudes into the duct. The use of such a two-part assembly enablesthe insert to be retro-fitted into an outer pipe which is part of anexisting fuel vent pipe system. The insert may be a pipe (such as acylindrical frame) or the insert may have a non-tubular structure.

The (or each) device may be positioned and shaped to enable the burstdisc to be visually inspected by looking along the duct through its openend along a centre of the duct, the centre of the duct not containingany of the devices. Alternatively the burst disc may be visuallyinspected by looking along the duct through its open end along aperipheral edge of the duct (i.e. not along its centre).

Typically each device extends into the duct from a base remote from theduct axis to an edge which is its closest point of approach to the ductaxis. The base of each device is at a distance R_(base) from the ductaxis, and the edge of each device is at a distance R_(edge) (which isless than R_(base)) from the duct axis.

A ratio [(R_(base)−R_(edge))/R_(base)] is typically greater than 0.1 andpreferably greater than 0.2. This ensures that each device protrudessufficiently far into the duct in order to inhibit the formation ofstanding waves.

The ratio [(R_(base)−R_(edge))/R_(base)] is typically less than 0.8 andpreferably less than 0.6. This ensures that the burst disc can be easilyinspected and the device does not significantly impede the passage offuel along the duct if the burst disc bursts.

Preferably the ratio [(R_(base)−R_(edge))/R_(base)] is less than 0.6 andgreater than 0.2.

The pipe body may have a non-circular sectional shape but morepreferably it has an inner wall which is substantially cylindrical.

Preferably the one or more devices consists of a prime number ofdevices.

The one or more devices may be offset from the burst disc so the burstdisc can burst into the duct without being impeded by the device(s). Theburst disc may be circular or any other shape.

Preferably a cross-sectional area of the duct does not change betweenthe different stations along the duct axis.

In some embodiments the (or each) device is a fin. In other embodimentsthe (or each) device is a wedge-shaped device The cross-sectional areaof the wedge-shaped device may decrease or increase as it extends alongthe duct towards the burst disc.

In some embodiments the (or each) device is a fin which extends into theduct from a base to an edge in the duct. Typically each fin has a thinstructure, so that a length of the fin from its base to its edge is muchgreater than its thickness. The fin may extend radially towards the ductaxis or at another angle.

An aircraft containing the fuel vent pipe typically comprises a fueltank coupled to the aircraft fuel vent pipe, the burst disc beingarranged to burst and enable fuel to flow from the fuel tank into theduct and out of the open end of the duct.

The aircraft fuel vent pipe may be positioned so that the open end ofthe duct is exposed to aerodynamic cross flow of air across the open endof the duct during flight of the aircraft—for instance the pipe may beinstalled in a lower surface of a wing of the aircraft.

Preferably the duct comprises a channel which enables the burst disc(which is in an intact state, in other words it has not burst) whichcloses the closed end of the duct to be visually inspected by lookingalong the duct through its open end along the channel.

The duct axis may be straight, or it might be slightly curved.

A further aspect of the invention provides a method of inhibiting theformation of standing acoustic waves in an aircraft fuel vent pipe, theaircraft fuel vent pipe comprising a pipe body containing a duct whichextends along a duct axis between an open end and a closed end; and aburst disc which closes the closed end of the duct, the methodcomprising inserting one or more devices in the duct, the device(s)being positioned or shaped to enable the burst disc to be visuallyinspected by looking along the duct through its open end, and to inhibitthe formation of standing acoustic waves in the duct. The device(s) andthe fuel vent pipe may be according to the first or second aspectinvention, as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theaccompanying drawings, in which:

FIG. 1 is a side view of an aircraft fuel vent pipe according to a firstembodiment of the invention;

FIG. 2 is a bottom view along a line A of the pipe of FIG. 1 lookinginto the duct;

FIG. 3 is a sectional view along a line B of the pipe of FIG. 1;

FIG. 4 is a sectional view along a line C of the pipe of FIG. 1;

FIG. 5 is a top view along a line D of the pipe of FIG. 1;

FIG. 6 is a side view of an aircraft fuel vent pipe according to asecond embodiment of the invention;

FIG. 7 is a bottom view along a line E of the pipe of FIG. 6;

FIG. 8 is a sectional view along a line F of the pipe of FIG. 6;

FIG. 9 is a sectional view along a line G of the pipe of FIG. 6;

FIG. 10 is a sectional view along a line H of the pipe of FIG. 6;

FIG. 11 is a cross-sectional view of an aircraft fuel vent pipeaccording to a third embodiment of the invention;

FIG. 12 is a cross-sectional view of an aircraft fuel vent pipeaccording to a fourth embodiment of the invention;

FIG. 13 is a bottom view of the fourth embodiment looking into the duct;

FIG. 14 is a cross-sectional view of an aircraft fuel vent pipeaccording to a fifth embodiment of the invention;

FIG. 15 is a bottom view of the fifth embodiment looking into the duct;

FIG. 16 is a front view of an aircraft;

FIG. 17 is a schematic side view showing a pipe installed on theaircraft; and

FIG. 18 is a schematic view of an aircraft fuel vent pipe with astanding wave.

DETAILED DESCRIPTION OF EMBODIMENT(S)

An aircraft fuel vent pipe 1 according to a first embodiment of theinvention is shown in FIGS. 1 to 5. The pipe comprises a pipe body witha cylindrical outer surface 2 and a cylindrical inner surface 3. Thepipe body contains a duct 4 which extends along a straight duct axis 5between an open end 6 and a closed end which is closed by a circularburst disc 7. The length of the duct between the burst disc 7 and theopen end 6 of the duct is typically of the order of 15-20 cm.

The pipe body carries three devices or fins 10-12 which are forgedintegrally as a single piece with the pipe body 2, 3. Each device 10-12extends radially into the duct from a base 10 d-12 d at the innersurface 3 of the pipe body to a free edge 10 c-12 c half way to thecentre of the duct as shown in FIG. 2. This enables the burst disc 7 tobe visually inspected by looking along the duct through its open endalong a channel at the centre of the duct, this channel not containingany of the devices 10-12.

The radial length of the devices (i.e. their length from the base 10d-12 d to the edge 10 c-12 c) must be sufficiently long to inhibit theformation of standing acoustic waves in the duct, whilst beingsufficiently short to enable the burst disc 7 to be easily inspected andnot significantly impede the passage of fuel along the duct if the burstdisc 7 bursts.

The duct axis 5 lies at the geometrical centre of the duct in the planeof the cross-section. When viewed in cross-section as in FIGS. 2 and 3,the base 10 d-12 d of each device is at a distance R_(base) remote fromthe duct axis 5 (the distance R_(base) being the furthest point of thedevice from the duct axis 5). Each device extends into the duct to adistance R_(edge) at its edge 10 c-12 c which is its closest point ofapproach to the duct axis 5. In the example of FIG. 3 the ratio[(R_(base)−R_(edge))/R_(base)] is 0.5, since each edge is half way tothe geometrical centre of the duct. This leaves a channel at the centreof the duct, not containing any of the devices 10-12, with a radiusR_(edge). In another example R_(edge) may be increased so that the ratio(R_(base)−R_(edge))/R_(base) is 0.25.

Each fin has a thin structure, so that a length of the fin from its base10 d-12 d to its free edge 10 c-12 c is much greater than its thickness(typically more than five or ten times greater).

Each device 10-12 extends along a majority (over 50%) of the full lengthof the duct between an outboard edge 10 a-12 a shown in FIG. 2 to aninboard edge 10 b-12 b shown in FIGS. 1 and 5. Each device 10-12 has anangular position about the duct axis 5 which changes along the duct axisto form a spiral shape. Due to this spiral shape, the devices 10-12collectively modify a sectional profile of the duct transverse to theduct axis so that the sectional profile of the duct changes betweendifferent stations along the duct axis as shown in FIGS. 2 to 5. FIG. 2is a bottom view of the pipe viewed along an arrow A in FIG. 1, FIGS. 3and 4 are cross-sections transverse to the duct axis 5 at differentstations B and C spaced apart along the length of the duct, and FIG. 5is a top view of the pipe viewed along an arrow D in FIG. 1. Note thatFIGS. 3 and 4 show the sectional profile of the duct at a single axialpoint or station, and for ease of illustration they do not show anyparts which lie behind the plane of the cross-section.

The outboard edges 10 a-12 a of the devices shown in FIG. 2 lie flushwith an annular rim 13 of the pipe body at the open end 6 of the ductand extend radially towards the duct axis 5. As the devices extendinboard towards the burst disc 7, the base 10 d-12 d at which they jointhe pipe body describes a spiral shape which rotates aroundthree-quarters of a full circle (that is, 270 degrees).

The outboard edges 10 a-12 a at the open end of the duct show in FIG. 2are positioned at angular (azimuthal) positions (relative to the ductaxis 5) of 0 degrees, 120 degrees and 240 degrees respectively. At theposition of FIG. 3 the devices 10-12 have rotated to angular positionsof 90 degrees, 210 degrees and 330 degrees respectively. At the positionof FIG. 4 the devices 10-12 have rotated to angular positions of 180degrees, 300 degrees and 60 degrees respectively. Finally, at the closedend of the duct shown in FIG. 5 the devices 10-12 have rotated toangular positions of 270 degrees, 30 degrees and 150 degreesrespectively. Thus although the sectional shape and area of the ductremains generally the same, the devices 10-12 modify the sectionalprofile of duct so that this shape rotates about the duct axis betweenthe different stations as can be seen by comparing FIGS. 2 to 5. Bymodifying the sectional profile of the duct so that it changes along theduct axis in this way, the devices 10-12 inhibit the formation ofstanding acoustic waves in the duct.

As shown in FIG. 1 the inboard edges 10 b-12 b of the devices are offsetaxially from the burst disc by a gap 9 so the burst disc 7 can burstinto the duct without being impeded by the devices.

In an alternative embodiment (not shown) rather than being flush withthe annular rim 13 of the pipe body, the outboard edges 10 a-12 a of thedevices are set back within the duct from the annular rim 13.

FIGS. 1 to 5 show a pipe 1 with three devices 10-12, but a similareffect can be achieved with only one device, five devices, or any othernumber of devices (preferably a prime number of devices).

The three devices 10-12 are forged integrally as a single piece with thepipe body 2, 3. Alternatively the three devices 10-12 may be formedintegrally as a single piece with the pipe body 2,3 by a process ofadditive layer manufacturing. Alternatively the three devices 10-12 maybe carried by a cylindrical frame (not shown) which is retro-fitted byinserting it into the duct and holding it in place with a cover plate(not shown) at the duct outlet.

FIGS. 6 to 10 show an aircraft fuel vent pipe 20 according to a secondembodiment of the invention. Many parts are the same as in the firstembodiment and in this case the same reference numbers are used. Insteadof containing three spiral devices 10-12, the pipe 20 contains threeplanar devices or fins 21-23, with the point at which each device 21-23joins the pipe body describing a straight line parallel with the ductaxis.

Each device 21-23 extends radially towards the duct axis 5 from theinner wall 3 of the pipe body into the duct, but stops short of the ductaxis 5 at the centre of the duct as shown in FIG. 7. The planar devices21-23 are positioned at different angular and axial locations to achievea similar effect to the spiral devices of FIG. 1.

The devices 21-23 each have outboard edges 21 a-23 a and inboard edges21 b-23 b shown in FIGS. 6 and 7. Each device 21-23 runs along only partof the length of the duct, and the devices are staggered atprogressively different axial points along the duct so that at any pointalong the axis of the duct there is only one device. Thus at the stationof FIG. 8 there is only one device 21 at an angular position about theduct axis of 0 degrees, at the station of FIG. 9 there is only onedevice 22 at an angular position about the duct axis of 120 degrees, andat the station of FIG. 10 there is only one device 28 at an angularposition about the duct axis of 240 degrees.

FIGS. 8-10 are cross-sections through the duct transverse to the ductaxis showing the sectional profile of the duct at that point, and forease of illustration they do not show any parts which lie behind theplane of the cross-section. As with FIG. 1, although the devices havethe same sectional shape (and hence the shape of the duct does notchange), they modify the sectional profile of duct so that its shaperotates about the duct axis between the different stations (due to thedifferent angular positions of the devices) as can be seen by comparingFIGS. 8-10. Thus the sectional profile of the duct is modified at afirst station along the duct axis by the device 21 but not the devices22, 23 (as shown in FIG. 8). Similarly the sectional profile of the ductis modified at a second station along the duct axis by the device 22 butnot the devices 21, 23 (as shown in FIG. 9). Similarly the sectionalprofile of the duct is modified at a third station along the duct axisby the device 23 but not the devices 21, 22 (as shown in FIG. 10). Bymodifying the sectional profile of the duct so that it changes along theduct axis in this way, the devices 21-23 inhibit the formation ofstanding acoustic waves in the duct. Although the devices 21-23 arestaggered along the duct axis without overlapping, there may be a degreeof overlap between them. Also, further devices may be added so that morethan one device is present at the first, second and third stations.

In the example of FIGS. 7-10 the ratio [(R_(base)−R_(edge)/R_(base)] is0.5, since the edge of each fin is half way to the geometrical centre ofthe duct. This leaves a channel at the centre of the duct, notcontaining any of the devices, with a radius R_(edge).

FIG. 11 shows an aircraft fuel vent pipe 50 according to a thirdembodiment of the invention. Many parts are the same as in the firstembodiment and in this case the same reference numbers are used. Thepipe body comprises an outer pipe with a cylindrical outer surface 2 anda cylindrical inner surface 3, and an insert 58 comprising an inner pipewith a cylindrical outer surface 51 and a cylindrical inner surface 52.The inner and outer pipes have annular flanges 53, 54, 55 which aresecured to each other by fasteners (not shown) after the insert 58 hasbeen inserted into the duct. The inner pipe carries fins 56, 57 whichprotrude into the duct and have a spiral shape like the fins in FIG. 1.The insert 58 can be retro-fitted into the fuel vent pipe of an existingaircraft. The flange 55 of the inner pipe lies flush with theaerodynamic outer surface of the lower skin 34 of the aircraft wing.

The duct axis 5 lies at the geometrical centre of the duct in the planeof the cross-section. When viewed in cross-section as in FIG. 11, thebase of each device is at a distance R_(base) remote from the duct axis5 (the distance R_(base) being the furthest point of the device from theduct axis 5). Each device extends into the duct 4 to a distance R_(edge)at its edge which is its closest point of approach to the duct axis 5.In the example of FIG. 11 the ratio [(R_(base)−R_(edge))/R_(base)] is0.5, since each edge is half way to the geometrical centre of the duct.This leaves a channel at the centre of the duct, not containing any ofthe devices 56, 57, with a radius R_(edge).

In the example of FIG. 11 these is an annular gap 59 between the outersurface 51 of the inner pipe and the inner surface 3 of the outer pipe.In an alternative embodiment these surfaces may abut each other so thereis little or no gap between them.

In an alternative embodiment instead of containing spiral devices 10-12or planar devices 21-23, the pipe may contain a number of wedge-shapeddevices, similar to vortex generators, which each protrude towards theduct axis 5 from the inner wall 3 of the pipe body into the duct butstop short of the duct axis 5 at the centre of the duct. Eachwedge-shaped device changes its cross-sectional shape and/or area as itextends along the duct axis, and inhibits the formation of standingacoustic waves in the duct. In this case, unlike the previousembodiments, the sectional shape and/or area of the duct will changebetween different stations along the duct axis (due to the change inshape and/or area of the wedge-shaped devices).

FIGS. 12 and 13 show a vent pipe 60 with two such wedge-shaped devices61, 62 which taper axially as shown in FIG. 12 and also taper inwardlyto sharp tips as shown in FIG. 13. In this case the tips of thewedge-shaped devices are directed upwardly so that their cross-sectionalareas decrease as they extend along the duct towards the burst disc 7.

FIGS. 14 and 15 show a vent pipe 70 with three such wedge-shaped devices71-73 which taper axially as shown in FIG. 14 and also taper inwardly tosharp tips as shown in FIG. 16. In this case the tips of thewedge-shaped devices are directed downwardly so that theircross-sectional areas increase as they extend along the duct towards theburst disc 7.

In the examples of FIGS. 12-15 the wedge-shaped devices each protrudeabout a third of the way into the duct, so the ratio[(R_(base)−R_(edge))/R_(base)] is about 0.33 (measured to the thickestpart of the wedge).

An aircraft 30 comprising an aircraft fuel vent pipe 1, 20, 50, 60, 70as described above is shown in FIG. 16. The aircraft has a pair of wings31 each containing a fuel tank 32 as shown in FIG. 17 coupled to thevent pipe 1, 20, 50, 60, 70 via a vent line 33. If the fuel tank 32 isoverfilled with fuel then the burst disc 7 in the vent pipe is arrangedto burst and enable excess fuel to flow from the fuel tank into the ductand out of the open end of the duct. For this reason the vent pipeextends downwardly and lies flush with the aerodynamic outer surface ofthe lower skin 34 of the wing. Thus the open end 6 of the duct isexposed to aerodynamic cross flow of air 35 across the open end of theduct during flight of the aircraft.

Although the invention has been described above with reference to one ormore preferred embodiments, it will be appreciated that various changesor modifications may be made without departing from the scope of theinvention as defined in the appended claims.

1. An aircraft fuel vent pipe comprising a pipe body containing a ductwhich extends along a duct axis between an open end and a closed end; aburst disc which closes the closed end of the duct; and one or moredevices in the duct, the device(s) being positioned or shaped to enablethe burst disc to be visually inspected by looking along the ductthrough its open end, and to inhibit the formation of standing acousticwaves in the duct.
 2. An aircraft fuel vent pipe according to claim 1,the device(s) being positioned or shaped to modify a sectional profileof the duct transverse to the duct axis so that the sectional profile ofthe duct changes between different stations along the duct axis.
 3. Theaircraft fuel vent pipe of claim 1 wherein the one or more devicescomprise a device with a sectional shape or an angular position aboutthe duct axis which changes between different stations along the ductaxis.
 4. The aircraft fuel vent pipe of claim 3 wherein the one or moredevices comprise a device with an angular position about the duct axiswhich changes between different stations along the duct axis.
 5. Theaircraft fuel vent pipe of claim 4 wherein the device has an angularposition which changes along the duct axis to form a spiral shape. 6.The aircraft fuel vent pipe of claim 1 wherein the one or more devicescomprise first and second devices which are located at different angularpositions about the duct axis and offset along the duct axis so that thesectional profile of the duct is modified at a first station along theduct axis by the first device but not the second device, and thesectional profile of the duct is modified at a second station along theduct axis by the second device but not the first device.
 7. The aircraftfuel vent pipe of claim 1 wherein the (or each) device extends from aninner wall of the pipe body into the duct.
 8. The aircraft fuel ventpipe of claim 1 wherein the pipe body comprises an outer pipe, and aninsert inside the outer pipe which carries the (or each) device. 9.(canceled)
 10. (canceled)
 11. (canceled)
 12. (canceled)
 13. (canceled)14. (canceled)
 15. (canceled)
 16. The aircraft fuel vent pipe of claim1, wherein each device protrudes into the duct from a base remote fromthe duct axis to an edge which is its closest point of approach to theduct axis.
 17. The aircraft fuel vent pipe of claim 16 wherein the baseof each device is at a distance R_(base) from the duct axis, the edge ofeach device is at a distance R_(edge) from the duct axis, and a ratio[(R_(base)−R_(edge))/R_(base)] is greater than 0.1 and preferablygreater than 0.2.
 18. The aircraft fuel vent pipe of claim 16 forwherein the base of each device is at a distance R_(base) from the ductaxis, the edge of each device is at a distance R_(edge) from the ductaxis, and a ratio [(R_(base)−R_(edge))/R_(base)] is less than 0.8 andpreferably less than 0.6.
 19. The aircraft fuel vent pipe of claim 1wherein the (or each) device is a fin.
 20. The aircraft fuel vent pipeof claim 19 wherein the (or each) device is a fin which extends into theduct from a base to an edge in the duct.
 21. The aircraft fuel vent pipeof claim 20 wherein a length of the fin from its base to its edge isgreater than its thickness.
 22. The aircraft fuel vent pipe of claim 19,wherein the (or each) device is a planar fin.
 23. The aircraft fuel ventpipe of claim 1 wherein the (or each) device extends radially into theduct towards the duct axis.
 24. The aircraft fuel vent pipe of claim 1wherein the (or each) device is a wedge-shaped device which changes itscross-sectional area as it extends along the duct.
 25. An aircraft fuelvent pipe comprising a pipe body containing a duct which extends along aduct axis between an open end and a closed end; a burst disc whichcloses the closed end of the duct; and one or more devices in the duct,the device(s) being positioned or shaped to enable the burst disc to bevisually inspected by looking along the duct through its open end, andto modify a sectional profile of the duct transverse to the duct axis sothat the sectional profile of the duct changes between differentstations along the duct axis.
 26. (canceled)
 27. An aircraft comprisingan aircraft fuel vent pipe according to claim
 1. 28. (canceled) 29.(canceled)
 30. A method of inhibiting the formation of standing acousticwaves in an aircraft fuel vent pipe, the aircraft fuel vent pipecomprising a pipe body containing a duct which extends along a duct axisbetween an open end and a closed end; and a burst disc which closes theclosed end of the duct, the method comprising inserting one or moredevices in the duct, the device(s) being positioned or shaped to enablethe burst disc to be visually inspected by looking along the ductthrough its open end, and to inhibit the formation of standing acousticwaves in the duct.